Why do Jews often so closely resemble the members of the nationalities amongst whom they have been living for only a few hundred years? How do features spread through such an exclusive population so quickly to the extent that it is easier to identify on sight national origin (with some degree of accuracy) than religious affiliation?

(I think this question might be founded on faulty assumptions about any subset of {genetics; statistics; physiology; Judaism}. If so, please correct.)

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5 Answers
5

I suspect that its a combination of some superset of the following factors:

Intermarriage and conversion bringing in genetic material from the host population.

Diet affecting people's hardiness, skin color, etc.

Standard of dental care affecting people's facial structure and whether they like to open their mouths.

Environmental factors, including sun exposure, which could affect skin color as well as things like hair loss.

Cultural norms for facial expressions and body language. I suspect that people from Moscow hold their faces like other Muscovites and differently, in a subtle way, from people from Cleveland or Singapore.

As a demonstration of the importance of my four proposed non-hereditary factors, I submit that Ashkenazic Jews who grew up in Israel look subtly but noticeably different from those who grew up in the US, even if they came from the same family origins, going back just a generation or two.

Another example: There's a guy I work with who was born in India. On his employee ID photo, which is from a few years ago, he looks incredibly more Indian than he does now, almost like a different person. I think part of it is that he's put on a little weight , but there's something harder to put my finger on that I'll bet is an American attitude's effect on how he holds his face. I saw a photo in his cubicle that was probably taken between his ID and now, and his "Indianness" seems in between to me.

The geneticist Luigi Cavalli-Sforza described racial features as "physiological frosting." Rambam Kilyaim 3:1 -- "you can have two types of plant that look different, but are really the same species, but they changed over time because they grew in different locations and conditions." Tiferes Yisrael on Sanhedrin 4:5: when the Mishna credits God with the amazing variety of people's different features, that means within a given race. As the variety between races is just caused by living in different places for a long enough time.
– ShalomMar 18 '10 at 13:48

1

I would tend towards a 99-100% genetic explanation, with almost no contribution from cultural/diet/environment. Those may indirectly explain the differences between races evolving over thousands of years, but the fact that Syrian Jews all born and bred in Brooklyn are still easily distinguishable from their Ashkenazi neighbors is due to genetics. And within the genetic answer, my hunch is that conversion historically played a very small role, while intermarriage and other, more violent variants were the dominant factor in genetic mixing.
– JeremyMar 18 '10 at 17:50

There are many pure bred syrians and Yemenites with green or blue eyes. The genetic potential is there. There are environmental factors and genetic factors, some that are independent of eachother and others that have some mysterious interplay. We just do not know enough yet to determine the conclusive answer to this question.
– YahuMar 18 '10 at 20:46

3

There might also be an element of "natural selection" here -- people who have features that are considered more desirable in the host society might have an easier time finding a shidduch, or getting a decent job, etc. Over many generations this could result in a subtle shift of the genetic "baseline".
– DaveSep 29 '10 at 1:58

There is a Gemara where Hillel was asked about facial features of different nationalities and responded that the location effected them. Similarly we see by animals that those that are in very cold environments have thicker furs.

The genetic potential is there. There are environmental factors and genetic factors, some that are independent of eachother and others that have some mysterious interplay. We just do not know enough yet to determine the conclusive answer to this question.

Where other mechanisms are nonviable or insufficient to explain changes in appearance, epigenetic mechanisms may be partially responsible for adaptation of local phenotypic traits by migratory populations within as little as one or two generations.

This may explain the fact that Jews throughout world are generally physically indistinguishable from non-Jews of their host countries despite the genetic evidence that suggests "that most Jewish communities have remained relatively isolated from neighboring non-Jewish communities during and after the Diaspora." (Hammer, et al.)

The Mishna (Kilaim 8:6) spoke almost 2000 years ago about micro-evolution: "Rabbis: the wild ox is a species of "behema", Rabbi Yossi says: it is a species of "chaya", Bartenura: "The Rabbis hold it used to be domesticated but it fled civilization and eventually transformed into the wild ox specie".

There he brings evidence that the DNA code has been found to change itself to adapt to the environment. ie NOT by "random" mutations, the dna reprograms itself

According to this, you can have the genetic code change even without any intermarriage. The facial features such as hair color or eye color, skin etc. change because they are advantageous in the climate of the host nation.

the other possibility is that this mechanism is not through genes modifying themselves but rather through the activation of dormant genes through some built-in mechanism which senses when to active them.